1. Field of the Invention
The present invention generally concerns optical data storage, coherence and statistical optics, volume holographic gratings, and holographic optical elements.
The present invention particularly concerns a volume holographic element, and the use of same for the simultaneous optical stamping of data into multiple layers of a three- dimensional bit-oriented optical medium that exhibits a suitable sensitivity threshold.
2. Description of the Prior Art
2.1 Stamping of Optical Disks
Owing to the relative economy of the medium, and the ease of reproduction, optical discs are currently (circa 2000) the preferred method for distribution of large quantities of information. Computer software, music, and video are among the most popular forms of data encoded on digital optical discs.
Unfortunately, theoretical limits in terms of the capacity of the planar, single-recorded-layer, optical disc are quickly being reached. Despite the small gains that may be made from alternative laser sources and improved coding techniques, it is becoming more and more apparent that a new storage technology will be necessary as the successor to the reflective optical disc.
There are several new optical technologies currently being investigated including multi-layer reflective media, volume holography, two-photon storage, near-field recording, and others. Although many of these technologies have shown promise as being re-writable and having very high data densities, none of these technologies have retained the one main advantage of the standard optical disc which is its ease of production.
For example, some 4.7 GB of information on a Digital Versatile Disc, or DVD, is presently (circa 2000) mechanically stamped onto each disc, permitting a fabrication line to produce discs at a rate of 1 every 12 seconds. No other advanced optical storage system currently under investigation today has exhibited a mass replication process as simple and practical as that for optical discs.
2.2 Recent Developments in Volume Optical Materials
Meanwhile, recent advances in non-linear optical materials have stimulated interest in the development of three-dimensional memory technologies. In particular, demonstrations have been made using polymeric materials that exhibit a non-linear optical response for multi-layer (volumetric) data storage. See J. H. Strickler and W. W. Webb,xe2x80x9cThree-dimensional optical data storage in refractive media by two-photon point excitation,xe2x80x9d Opt. Lett. 16, 1780-1782 (1991); S. Kawata and A. Toriumi,xe2x80x9cThree-dimensional Optical Memory Using Photopolymer, Photorefractive Crystals, and Photochromic Materials,xe2x80x9d Proc. SPIE 3109, 174-180 (1997); and M. M. Wang and S. C. Esener,xe2x80x9cThree-dimensional optical memory in fluorescent dye-doped photopolymer,xe2x80x9d submitted to Applied Optics (Dec. 12, 1998).
Demonstrations have been also been made of the use of polymeric materials for three-dimensional micro-component fabrication. See S. Maruo, O. Nakamura, and S. Kawata,xe2x80x9cThree-5 dimensional microfabrication with two-photon-absorbed photopolymerization,xe2x80x9d Opt. Lett. 22, 132-134 (1997).
A key factor which will determine whether these new technologies will become practical for the mass production of memory devices that are already recorded at the time of sale will be the speed and economy at which these devices may be fabricated.
Two possible approaches to increasing the fabrication rate are, first, to increase the sensitivity of the materials to allow high speed serial raster recording of the desired data, and, second, to attempt to record data in some degree of parallelism. The present invention is concerned with second possibility, specifically with three-dimensional data stamping, meaning the recording of data in parallel into each of multiple regions, normally planes, of a three-dimensional bit-oriented volume optical medium.
Any ability to emplace voluminous digital data in parallel, and all at the same time, within the three-dimensional volume of a optical memory, normally an optical disk, as a type ofxe2x80x9cdata stampingxe2x80x9d would be very useful in many areas. Data stamping is projected to be useful for data recording in fields including (i) entertainment, such as audio, video, and multimedia information distribution, video games; (ii) computer data storage, such as software distribution, large database, data archiving, 3D video; (iii) information processing, such as image recognition, voice recognition, relational databases; and (iv) optoelectronics, such as fiber optics, free-space optics, optical processing and communication.
Additionally, if the data stamping transpires at sufficiently fine spatial resolution, and if the process is not adversely affected when extensive spatial volumes are written entirely to one binary state or the other, then data stamping can be envisioned as a means of creating three-dimensional objects, complete with cavities and voids if any be present. The object created by three-dimensional data stamping might appear, proper materials being chosen, as if encased in glass or might actually, proper materials being chosen, become separable from a surroundingxe2x80x9cunwrittenxe2x80x9d matrix, and usable as axe2x80x9cformedxe2x80x9d object.
The present invention contemplates a new 3D optical storage technology which has the potential to replace standard optical discs as the next generation of distributable optical media. The invention is manifest in a new technique by which digital data may be optically stamped, in a process calledxe2x80x9cdata stampingxe2x80x9d, simultaneously into (i) bit-oriented (ii) multiple layers within (iii) the 3D volume of a volume optical memory, typically a (thick) disc. The virgin optical memory medium, prior to being data stamped, is typically as a continuum, and without any content or indexing (optical or otherwise) whatsoever. The data stamping xe2x80x9cwritesxe2x80x9d the entire volume of the media simultaneously in one operation, leaving (most typically) multiple layers of data each of which layers is fully organized into addressable voxels.
Multi-layer volume optical memories, particularly in the form of optical discs, have the advantage of being able to significantly increase the data capacity of a single disc without incurring a large increase in access time. The technique of the present invention is directed to recording digital data just as quickly, cheaply and reliably as single-layer planar optical discs are recorded today (circa 2000). Components similar to those used in the technique of the present invention have been in use for many years, and have already been demonstrated to possess all the necessary performance requirements for use in the present invention.
1. A (i) Particular Holographic Element for (ii) Data Stamping (iii) A Volume Optical Memory Made From a Non-linear Optical Medium
The present invention has several parts. In aggregate, the present invention teaches a multi-step process. First, an appropriate special holographicxe2x80x9cdata stampingxe2x80x9d element is generated. Then, second, this generated holographic data stamping element is used and re-used for the simultaneous optical data stamping of multi-layer data into the volume of a three-dimensional optical medium. Importantly, this optical medium must, and does, exhibit a suitable non-linear responsexe2x80x94manifested as a sharp radiation sensitivity thresholdxe2x80x94so as to permit the recording of large numbers of data bits in parallel.
Each of the processes of (i) generating the special holographic data stamping element (and the holographic element so generated), (ii) performing the optical data stamping by use of the generated holographic element, and (iii) using an appropriately non-linear three-dimensional optical medium, constitute separate parts of the present invention.
1.1 Generating a Holographic Data Stamping Element
The (i) generating of the holographic data stamping element, and the holographic data stamping element so generated, are special both for (a) how the element is generated, and (b) what the element contains. If the holographic element is to xe2x80x9cstamp dataxe2x80x9d in each of multiple planes, then a practitioner of the holographic arts might recognize that, by way of example, a recordable holographic material, such as the photopolymers of the DuPont Corporation specifically preferred for this task, could be and should be placed in the optical path of a 4-f imaging system, in location either before or after the image plane. The data mask of a single layer would then be recorded in the hologram. The position of the image plane would then be shifted axially and the next data mask recorded.
This rudimentary prior process is insufficient, and possessed of a serious flaw. Since the light that is imaging all of the planes (in turn) is coherent, coherent interference between the data planes will cause a large amount of noise in the reconstruction. To solve this problem the present invention employs several angularly-multiplexed reference beams instead of just one.
Moreover, these reference beams should be, and, in accordance with the present invention, preferably are, mutually incoherent so as to avoid coherent artifact noise.
The holographic data stamping element ultimately generated contains multiple images of planes of data, each image plane having been generated by (i) multiple (ii) incoherent (iii) angularly-separated reference beams. (Note that one reference beam does not generate one image plane; multiple reference beams generate each image plane.)
1.2 Using the Holographic Data Stamping Element
In accordance with the present invention the generated holographic data stamping element is used to record the same data with which it has been impressed into a volume optical medium, forming thereby a volume optical memory. The recording is done redundantly, with a number of different reference beams. It is done all at the same time in axe2x80x9cdata stampingxe2x80x9d operation. The optical data content of each layer to be recorded in the medium of the optical memory will necessarily be imaged in focus at only one plane (in depth) within the memory. However, the optical power intended for each layer will by necessity propagate through all layers in the media. The light from all other layers propagating through any given layer will be manifest as noise at that layer.
Now, by reference to the preceding section 1.1, it will be understood that a higher degree of incoherency in the readout of the holographic data stamping element is achieved by reconstructing the hologram with all of a number of illuminating reference beams simultaneously, each reference beam being (i) at a different reference angle, and being derived from (ii) a separate coherent light source which is incoherent with respect to the light source of every other reference beam. As the number of mutually incoherent reference beams is increased the magnitude of the coherent noise will go down until the only evidence of neighboring data layers at any given layer will be a background optical noise of nearly uniform intensity.
However, this uniform intensity background noisexe2x80x94which is an achievement of the present inventionxe2x80x94is also a drawback. This is because this noisexe2x80x94which is consequence not only of the data stamping method of the present invention but any method optically writing more than one layer at one timexe2x80x94significantly reduces the contrast ratio observed at any given layer.
Certain optical media that exhibit highly non-linear recording characteristics are, however, able to remove this background from the signal during the recording process. In particular, the dye- doped photopolymer media that is the subject of the related patent application, and that is preferred for use in the method of the present invention, exhibits a sharp step-like threshold in its recording intensity curve. See that related application, incorporated herein by reference. See also M. M. Wang and S. C. Esener,xe2x80x9cThree-dimensional optical memory in fluorescent dye-doped photopolymer,xe2x80x9d submitted to Applied Optics (Dec. 2, 1998), op cit.
Intensities below the threshold of the dye-doped photopolymer do not stimulate recording. However, intensities just beyond this threshold exhibit a high recording rate. By setting the signal intensity of the data layer above the recording threshold of the media and the background intensity below the threshold, the desired data may be recorded with a high contrast ratio.
2. Generating a Master Hologram Suitable for Data Stamping
Accordingly, in one of its aspects the present invention is embodied in a method of generating in a first optical medium a master hologram containing information suitably imaged simultaneously into multiple bit-oriented layers within a volume of a second optical medium. The master hologram is thus suitable for holographic stamping of multi-layer bit-oriented optical medium.
The preferred method consists of redundantly generating the master hologram in the first optical medium by use of a plurality m of separate reference beams, each beam bearing information suitably recorded in one or more of a plurality n layers of the second optical medium that are distinct from layers in which information in all other beams is suitably recorded.
The redundantly generating is preferably by use of a plurality m of different reference beams. The m different reference beams may differ in angular separation and/or by being incoherent.
The present invention is also embodied in an optical hologram produced by this method.
3. Holographically Stamping a Multi-layer Bit-oriented Optical Medium
In another of its aspects the present invention is embodied in a method of holographically stamping a multi-layer bit-oriented optical medium.
The method consists of recording information within a plurality n layers of the volume of the blank optical medium with m separate recording beams. The recording beams are produced by simultaneously illuminating a master hologram with m reference beams simultaneously. The recorded optical medium exhibits highly non-linear recording characteristics sufficient so that a recording beam holographically focused to a target layer possesses intensity sufficient to write the layer while the cumulative intensities at all layers arising from the collective beams not focused to the target layer remain insufficient to write the target layer.
By this procedure, performed on this optical medium, the entire optical medium may be optically written, or stamped, in its multi-layer three dimensional volume entirely at one time.
The recording is preferably by use of a plurality m of separate reference beams that are different by being (i) mutually incoherent and/or (ii) angularly separated, and preferably both.
The first optical medium exhibiting highly non-linear recording characteristics is preferably unresponsive to beam intensities below a threshold, these beam intensities being insufficient to write the medium, while beam intensities just beyond this threshold are sufficient to write the medium. This makes that when a recording beam holographically focused to a layer will possess intensity sufficient to write the layer while the cumulative intensities at all layers arising from the collective beams not holographically focused to that layer will remain insufficient to write the layer.
The recording is more preferably within a dye-doped photopolymer medium exhibiting the highly non-linear recording characteristics.
4. Holographic Data Stamping
The (i) generating of a master hologram suitable for data stamping and (ii) the holographically stamping of a multi-layer bit-oriented optical medium may usefully be combined as an integrated system, and method.
The method may be considered an improvement upon more rudimentary prior methods where a master hologram within a first optical medium containing information to be placed within the volume of a second optical medium is first generated, and then this generated master hologram is used to record the information within multiple layers of the three dimensional volume of a (blank) second optical medium.
In accordance with the present invention, the generating is improved by redundantly generating the master hologram in the first optical medium. The redundant generation transpires by use of a plurality of m separate reference beams, each beam bearing information suitably recorded in one or more of a plurality n layers of the second optical medium.
The using is also improved: generation of the master hologram recording the information within the n layers of the volume of the blank optical medium by is improved by use of m recording beams that are produced by simultaneously illuminating the master hologram with m separate reference beams.
The redundantly generating is more preferably by use of the plurality m separate reference beams that are different by being (i) angularly separated and/or (ii) mutually incoherent. The reference beams are more preferably both (i) angularly separated and/or (ii) mutually incoherent.
Likewise, the using of the generated hologram is more preferably by simultaneously illuminating the master hologram with the m different reference beams that are themselves different by being (i) angularly separated and/or (ii) mutually incoherent, and preferably both (i) angularly separated and (ii) mutually incoherent.
The using is preferably so as to record information within a second optical media exhibiting highly non-linear recording characteristics. Namely, recording beam intensities below a threshold are insufficient to stimulate recording while recording beam intensities just beyond this threshold exhibit a high recording rate. The intensity of the recording beams are set above a recording threshold of the second optical medium where the recording beams are focused on layers while background intensity within the second optical medium remains below this threshold, making that the second optical medium is recorded with a high contrast ratio.
The using of the master hologram is still more preferably to record information within a dye-doped photopolymer medium that exhibits the highly non-linear recording characteristics.
A least two combinations of reference beams are suitable. The m separate reference beams as do redundantly generate the master hologram may be used to record each of the n layers of the volume of the blank second optical medium. Alternatively, each of the n layers of the second optical medium is recorded with the m separate reference beams, resulting in a total of m x n separate reference beams.
5. Optical Holograms for Data Stamping, and Data-Stamped Optical Memories
In still yet another of its aspects the present invention is embodied in a an optical hologram suitable to holographically stamp a multi-layer bit-oriented first optical medium. The optical hologram is characterized in that (i) information within the optical hologram is organized for bit-oriented recording of another multiple layers within the three-dimensional volume of another, second, optical medium, while (ii) the optical hologram has itself been generated by use of a plurality m of separate reference beams, each beam bearing information suitably recorded in one or more of a plurality n layers of the second optical medium that are distinct from layers in which information in all other beams is suitably recorded.
The optical hologram is preferably so generated by use of m separate reference beams that differ in (i) angular separation and/or (ii) by being mutually incoherent. The reference beams preferably differ in both (i) angular separation and (ii) by being mutually incoherent.
The optical hologram may be generated (i) so as to be suitably redundantly illuminated by the same m separate reference beams in order to record each of the n layers of the volume of the blank second optical medium, or, alternatively, (ii) so as to be suitably redundantly illuminated by a different m reference beams in order to record each of the n layers of the volume of the blank second optical medium.
In a similar manner, the present invention is also embodied in a multi-layer bit-oriented information-containing volume optical memory characterized for having been made by a process of data stamping, meaning that all the information that is within the memory was placed into all the multiple layers thereof all at the same time. As a consequence of this data stamping, the volume optical memory is optically readable in layers, each layer by a plurality of reference beams that are typically so many in number as there are layers.
In particular, the multi-layer bit-oriented information-containing volume optical memory is preferably made by use of a plurality m of separate reference beams, each beam bearing information that was recorded in one or more of a plurality n layers of an optical medium of the memory. The m separate reference beams may differ in angular separation and/or by being mutually incoherent.
The memory may be suitably redundantly illuminated by a same m separate reference beams in order to read each of n layers within the volume of the memory or, alternatively, by a different m reference beams in order to record each of the n layers within the volume of the memory.
These and other aspects and attributes of the present invention will become increasingly clear upon reference to the following drawings and accompanying specification.